The present invention relates to holographic data storage.
Many different types of data storage media have been developed to store information. Traditional media, for instance, include magnetic media, optical media, and mechanical media to name a few. Increasing data storage density is a paramount goal in the development of new or improved types of data storage media.
In traditional media, individual bits are stored as distinct mechanical, optical, or magnetic changes on the surface of the media. For this reason, medium surface area may pose physical limits on data densities.
Holographic data storage media can offer higher storage densities than traditional media. In a holographic medium, data is stored throughout the volume of the medium rather than the medium surface. Moreover, data can be superimposed within the same medium volume through a process called shift multiplexing. For these reasons, theoretical holographic storage densities can approach tens of terabits per cubic centimeter.
In holographic data storage media, entire pages of information can be stored as optical interference patterns within a photosensitive optical material. This can be done by intersecting two coherent laser beams within the optical material. The first laser beam, called the object beam, contains the information to be stored; and the second, called the reference beam, interferes with the object beam to create an interference pattern that can be stored in the optical material as a hologram. The object beam and reference beam ordinarily follow separate optical paths.
When the stored hologram is later illuminated with only the reference beam, some of the reference beam light is diffracted by the hologram interference pattern. Moreover, the diffracted light creates a reconstruction of the original object beam. Thus, by illuminating a recorded hologram with the reference beam, the data encoded in the object beam can be reconstructed and detected by a data detector such as a camera.
In addition to the high data densities that can be achieved in holographic data storage media, holographic data storage media also present other potential advantages over more traditional media. For instance, because laser beams are used to detect the data, the use of mechanical components may be minimized. Moreover, eliminating mechanical components can increase data access times. Thus, holographic data storage systems can have access times as fast as 10 xcexcs.
Nevertheless, even though holographic data storage media present attractive data storage density capabilities and fast access times, creating a marketable product has proven difficult. Among other things, the relatively high cost of system components, and system configuration constraints have undermined the ability to create holographic data storage systems that can compete in the market with existing data storage technologies.
The invention presents the concepts of self-referenced holographic data recording. In exemplary embodiments, the invention comprises methods of recording holograms on holographic data storage media; media produced by such methods; and self-referencing holographic data storage system has a single optical path directed towards a holographic recording medium.
In one exemplary embodiment, a method of recording data in a holographic recording medium comprises illuminating the medium with a data encoded first optical beam, optically directing a zero frequency Fourier component of the first optical beam to create a second optical beam, and illuminating the medium with the second optical beam. For example, the medium can be placed in proximity to a Fourier transform plane of the optical system. Then, while illuminating the medium with a first optical beam, the Fourier component of the first optical beam can be directed to create the second optical beam which may comprise a holographic reference beam.
The first optical beam and the second optical beam may follow the same optical path. For instance, the path of the reference beam may be coaxial with the path of the of the object beam. In other embodiments, the invention comprises holographic data storage media recorded using these methods.
Optically directing the zero frequency Fourier component of the first optical beam may comprise positioning a mirror after a media plane to reflect the zero frequency Fourier component of the first optical beam. Alternatively, optically directing the zero frequency Fourier component of the first optical beam may comprise positioning a lens before the media plane to refract the zero frequency Fourier component of the first optical beam. If a mirror is used to optically direct the zero frequency Fourier component of the first optical beam, the mirror may be a flat mirror positioned at the center of a Fourier plane. Alternatively, the mirror may be a concave mirror positioned a distance beyond a Fourier plane, or a convex mirror positioned a distance before a Fourier plane.
In still other embodiments, the invention comprises a holographic data storage system. The system may include a laser that produces laser light and a collection of optical elements. The optical elements may create an object beam and a reference beam from the laser light and the reference beam may be created from a zero frequency Fourier component of the object beam. The system may include a data encoder for encoding data in the object beam and the system may also include a holographic medium. Moreover, the holographic medium may contain a hologram created by the interference of the reference beam and object beam. The system may further include a data detector for detecting data in the reconstructed hologram when the reference beam illuminates the medium.
In the system, the data encoder may be a spatial light modulator, and the data detector may be a camera. The hologram may be generated from an object beam constructed of a pixel array that is a data encoded bit map. The collection of optical elements may define an optical path, and the object beam and reference beam may both follow the common path of optical elements. In some embodiments, the common optical path is the only optical path in the system.
Additional details of these and other embodiments are set forth in the accompanying drawings and the description below. Other features, objects and advantages will become apparent from the description and drawings, and from the claims.